Silver nanowires, which inherit excellent conductivity and thermal conductivity of block silver, are one of the best choices for efficient transmission of electrical signals. Moreover, as one-dimensional precious metal nanomaterials, the silver nanowires, which have surface plasmonic effects, can excite surface plasmonic polariton (SPP) mode propagating along the interface between the metal and the medium, thus confining the light beam within the nanometer scale and breaking the optical diffraction limit, causing a great application potential in the nanoscale optoelectronics field. Furthermore, interacting with light the silver nanowires can excite unique localized surface plasmon resonance (SPR) and thus cause strong absorption in UV and visible light band, which is great significant for the applications of the thin film devices and non-linear enhancement. Such characteristics and application directions of silver nanowires are related closely to the diameter, to be specific, the nanowires with thick diameter are more suitable for the field of subwavelength waveguide devices and those with thin diameter are mainly applied in the field of thin film devices.
For waveguide devices, limited by the optical diffraction limit, the mode size of light in traditional medium waveguide is generally between several to dozens of microns. Later, waveguides made of materials with high diffraction were able to limit the mode size to several hundreds of nanometers. However, such scale still fails to reach the scale of integrated electronic chips, which makes it difficult to achieve nanoscale integrated optoelectronic system and optoelectronic hybrid system. Silver nanowire waveguides based on SPP excite the surface electromagnetic wave transmitting between the interface of metal and medium and can limit the light in nanoscale space. Such limiting ability is stronger than that of traditional dielectric waveguide and supports a smaller mode spot. Nanowires waveguide essentially break through the limitation of optical diffraction limits, thus significantly improving the integration of optical devices and being one of key techniques of achieving the next generation of ultra-large-scale integrated photonic chip system; at the same time, as the silver can transmit both optical signal and electrical signal, the surface plasmon waveguide can also be used to achieve a photoelectric hybrid system. The transmitting efficiency of light in silver nanowire waveguide is closely related to the diameter of nanowires. The thicker the diameter of nanowires, the more the energy constrained inside the nanowires, the less the outside energy of evanescent wave and the further the transmitting distance. At the same time, the transmitting mode of light within nanowires is closely related to the diameter. Owning to the larger consumption relative to fundamental mode, high-order mode must be achieved by the nanowires with thick diameter. In addition, nanowires with thick diameter can also achieve more novel applications. When applied in the field of thin film devices, the scattering effects of the thick nanowires contributes to conductive reflective thin films; when applied in the field of new energy, nanowires can play positive roles on the backplane of solar cells such as promoting the secondary absorption of light, and so on. The length of silver nanowires should be also increased while thickening the diameter of nanowires. Long nanowires can interact with the gain medium to achieve long range transmission and further diversify the function of subwavelength waveguide devices. However, the current thickest diameter of silver nanowires can only reach two or three hundred nanometers with the length of one hundred microns. Waveguides made by such nanowires cause shorter transmission distance, large transmission loss of high-order mode and single function, which makes it hard to achieve the practical application of nanoscale optical waveguides to the integrated optoelectronic devices and photoelectric hybrid circuits. Thus, it is the only way to prepare nanowires with thicker diameter and longer length for propelling further forward development of subwavelength waveguide devices and implementing applications.
In the field of thin film devices, the thin film devices prepared by the traditional material, indium tin oxide (ITO), have several unavoidable disadvantages such as poor flexibility, fragile, costly and resources shortage, resulting in being difficult to satisfy the demands of the development of flexible electronic devices. The silver nanowires interacting with light excite surface plasmon resonance effects, which show as two characteristic peaks in the transverse and longitudinal directions. When the nanowire is longer, the longitudinal characteristic peak red-shifts to infrared range only with the transverse peak in visible light band occurs, thus having high transmittance in the visible light band. When the silver nanowires with high transmittance are interconnected to form network, the thin film devices are allowed to have excellent electrical conductivity; besides, the nanowires have the advantages of excellent flexibility, stable chemical performance, simply prepared, low-cost and applicable for industrial production. Therefore, silver nanowires are considered the most potential alternative for ITO to prepare high-quality thin film devices. The performance of thin film devices based on silver nanowires depends on several important parameters: haze, transmittance and sheet resistance. Haze, an important parameter that describes the degree of fuzzy of things that people see, is the bottleneck restricting the current development of silver nanowire thin film devices. Haze is decided by the scattering ability in broad spectrum range of the nanowires, which is related closely to the diameter of nanowires. Silver nanowires with thick diameter have strong scattering effect, causing high haze and the sharp decrease of the clarity of thin film devices; when the diameter of nanowires is about 10 nm, the haze problem of thin film devices can be effectively improved so as to reach the haze level for ITO based thin film devices. Transmittance, a key parameter to characterize the transparent properties of thin film devices, is dependent on the SPR effect of nanowires. With the decrease of the diameter, the resonance peak blue shifts leading to higher transmittance. Transmittance also varies with the aspect ratio of silver nanowires. When silver nanowires interconnected to a thin film, light may be partly blocked out by a great deal of nodes formed by the intersections among nanowires. Therefore, the longer the aspect ratio, the less the nodes and the better the performance of transmittance. Sheet resistance used to indicate the conductivity of the thin film devices increases with the decrease of the diameter of nanowires, that is to say, the thinner the nanowires, the larger the sheet resistance, so that the poorer the conductivity. Thus, reducing the diameter and increasing the aspect ratio of silver nanowires are greatly conducive to improving the haze problem and transmittance of thin film devices, however, reduce the conductivity to some extent. Consequently, there is an irreconcilable contradiction among these three parameters. Theoretical studies have shown that the silver nanowires with the diameter of 10 nm can take in account above performances, which can ensure a good conductivity while effectively improving the haze of thin film. But the extant preparation method neither prepares nanowires with the diameter of 10 nm, nor achieves large-scale industrial production with high yield. Therefore, it is an inevitable choice to prepare nanowires with thin diameter and high aspect ratio for improving transmittance of thin film devices and solving the haze problem.
The current preparation method for silver nanowires is mainly Polyol method. The silver nanowires are prepared by one-step method where the silver nitrate is reduced by glyoxal, which is produced under high temperature by the decompensation of ethylene glycol, with the protection of polyvinylpyrrolidone and the supplementary effect of sodium chloride. This method achieves the controllable growth of nanowires by precisely controlling the yield and output of seeds. However, two phases of forming crystal seeds and longitudinal growth in one-step method go on simultaneously and interfere with each other. In the phase of forming crystal seeds, longitudinal growth along existed crystal seeds may increase the non-uniformity of the diameter; and in the phase of longitudinal growth, new isotropic seeds generated by self-nucleation may gravely block the production of nanowires and also consume the silver sources which should be used for longitudinal growth, leading to the decrease of the length and yield of silver nanowires. Once the amount of reaction is enlarged, the isotropic seeds formed by self-nucleation in the reaction system may severely retard the generation of nanowires so as to result in low yield nanowires or even being unable to obtain silver nanowires, which makes it incapable to achieve industrial production. In addition, the seeds are seriously susceptible to the uncontrollable factors such as the additions, purity of medicine, temperature, humidity and exposure degree to air during growth, thereby causing the key step, the phase of the formation of seeds, hard to be precisely controlled and the synthesis reaction difficult to be repeated. Many researchers have expanded their studies on one-step method. However, there are still many blocks to be solved in one-step method, which is inaccessible to meet the demands for controllable growth, industrial production and application of silver nanowires. Thus, it is necessary and urgent to improve or propose a preparation method for silver nanowires to break through the current range of diameter and obtain the silver nanowires with uniform diameter and longer length, which is the pressing problem to be solved immediately in the field of waveguide devices and thin film devices.